The invention relates to a process for the removal of dissolved components from aqueous solutions.
It is known in the art that liquid membrane emulsions may be used to remove dissolved substances from aqueous solutions; see, for example, U.S. Pat. Nos. 3,617,546; 3,637,488; and 3,779,907 which are incorporated herein by reference. The emulsion is characterized as having a dispersed or internal phase suspended in a continuous phase. The continuous phase is immiscible with the aqueous solution but is permeable to the dissolved substance. The dispersed phase, which usually is miscible with the aqueous phase, contains a reactant capable of reforming the dissolved substance in the dispersed phase. The emulsion is contacted with the aqueous solution, whereupon the dissolved substance permeates the continuous phase into the dispersed phase and is retained therein.
One way to achieve this is to convert the dissolved substance, after it has permeated into the dispersed phase, into a form in which it is incapable of permeating back through the immiscible continuous phase (i.e. the liquid membrane), such as by neutralization or by precipitation.
Another method is based on the preferential complexing action of the said dissolved substance in the continuous phase. The continuous phase may be specially adapted to assist the permeation of the dissolved substance. A complexing agent may be included in the continuous phase. This agent is capable of forming a complex soluble in the continuous phase with the dissolved substance. The complex then reacts with a reactant contained in the dispersed phase which converts the first complex to a second complex by replacing the dissolved substance in the complex such that the dissolved component remains in the dispersed phase. For example, to remove metal ions from an aqueous solution, an oil-soluble ion-exchange compound may be the complexing agent included in the continuous phase, in order to form said first complex. Hydrogen ion present in high concentration in the internal dispersed phase, displaces the metal ion in the complex to form said second complex. If the metal ion is copper, then a suitable ion-exchange compound is a mixture of .beta.-hydroxy benzophenone oxime and an .alpha.-hydroxy oxime which make up the proprietary mixture known as LIX64N marketed by General Mills Chemical Co. See U.S. application, Ser. No. 669,706 assigned to the same assignee as the present invention which is incorporated herein by reference.
In the conventional liquid membrane (LM) extraction process utilized in, say, the removal of copper from dilute mine water, the aqueous liquid is contacted with a water-in-oil emulsion where the continuous (oil) phase contains LIX64N and the encapsulated dispersed aqueous phas is the spent electrolytic cell liquid. Copper will permeate from the mine water, where it is present in low concentration, through the LIX64N containing oil "membrane" into the highly acidic internal dispersed phase where the copper concentration will build up to a high level. After the extraction has proceeded to the desired extent, the emulsion is allowed to settle away from the treated mine water which is returned to the leach pile for further extraction of mineral values. The spent emulsion is broken, one preferred method being a combination of centrifugation and agitation with excess internal aqueous phase as disclosed in copending Ser. No. 677,527, which is incorporated herein by reference. The internal aqueous broken out of the emulsion can now be taken to the electrolytic cells where a portion of the contained copper ions are recovered as metallic copper with a concommitant increase in solution acidity. The spent cell liquid and the oil phase separated in the demulsification step are now recombined and form the fresh emulsion which is recycled to the dilute mine water treating step.
The process as described has two principal functions.
1. to concentrate the copper in the liquid to simplify the electrolytic recovery operation (the concentration of copper in the dilute mine water is normally 0.5-2.5 g/l, while the cell liquid ranges from 30 g/l spent to 60 g/l rich electrolyte)
2. to separate the copper selectively from the large quantity of other metal ions, especially iron, present in the leach liquid (other ions are, for example, Mg++, Al+++, Fe++, Fe+++).
However, in the removal of metals from aqueous solutions by means of a liquid membrane (LM) formulation containing (a) an ion exchange resin or complexing agent in the continuous phase and (b) a recipient aqueous dispersed phase where the extracted ion can concentrate, the following problems may occur.
When certain formulations of membrane are used, the internal aqueous phase may swell due to entrainment of the aqueous solution or osmosis of water from the external aqueous solution to the internal dispersed phase. The swell occurring in the course of an LM treating step may depend on the mixing time. In one case, the degree of swell is a linear function of time. In a second case, swell increases exponentially with time, and in a third case, swell may approach a constant value asymptotically, which means it is only of significance during the initial period of agitation. The present invention is particularly applicable to systems which swell in accordance with the first two cases. Emulsion swell, which is usually expressed as percent increase in weight of internal dispersed phase per unit time of mixing, is objectionable in metal extraction, although it may be acceptable in other types of applications such as waste water treatment, where the emulsion may be used on a once-through basis. Since the metal extraction process is used for the purpose of separating and concentrating the dissolved metal ions, any dilution of the receiving aqueous phase is undesirable. The water thus introduced into the concentrated dispersed emulsion phase has to be removed again in subsequent processing. If, in addition, constituents of the aqueous solution feed other than water become incorporated into the emulsion and cause swell, then the purity of the recovered and concentrated dissolved substance is adversely affected, decreasing some of the benefits of the overall process. These and other objections to the swelling phenomena in certain cases become apparent on further discussion. There are systems where negative swell occurs probably due to osmosis of water from the dispersed internal phase to a highly ionic external feedstream being treated. The present invention also is useful in these systems since it will minimize said negative swell (shrinkage of emulsion).
In contrast to swell, there is a phenomenon occurring which is in the opposite direction. It has been found that in cases when a certain membrane formulation is used the dispersed concentrated phase slowly leaks into the aqueous phase being treated. Leakage is the slow and continuous breakup of the emulsion, resulting in the essentially constant addition of highly concentrated internal phase to the external aqueous phase being treated. This problem usually can be resolved by using certain additives to strengthen the membrane. In the metal extraction case, the "spilled"material is continuously absorbed by the remaining emulsion, thus adding a load to the permeation duty. Leakage results in low apparent extraction rates at the low Cu-concentration end of the operation.
As long as the copper concentration in the aqueous phase being treated is high, this leakage, even if it occurs with certain emulsions, usually is unimportant. However, as the Cu-concentration gets into the lower ranges, the leakage, if it occurs, becomes a larger fraction of the extraction load and may limit the degree of Cu cleanup which can be achieved. The rate with which a dissolved constituent can be extracted into a liquid membrane emulsion can be expressed as a first order rate equation with concentration over a considerable concentration range. However, if leakage occurs, there is a drop-off in the apparent first order permeation rate constant with time at the low concentration end of the process resulting in longer residence time than calculated by first order kinetics being required in the cleanup step.
It has now been found that these limitations found in certain cases can be effectively overcome, and the overall operation improved by the present invention, which combines liquid membrane extraction and conventional solvent extraction.
Solvent extraction is the separation of the constituents of a liquid solution by contact with another insoluble liquid, if these substances can distribute themselves in both of these two liquid phases. In general, to gain rapid extraction rate, the two liquid phases are intimately mixed so that one phase forms fine droplets dispersed in the other phase. Another way is to contact the two liquid phases by sending them co-currently or counter-currently into a column packed with inert solid particles, such as glass beads. The purpose of using a packed column in extraction is to achieve countercurrency and to increase the residence time of and the interfacial area between the liquid phases, thereby achieving a high extraction rate.
As a result of this improvement (1) metal extraction to very low levels can be achieved; (2) swell of the internal phase can be minimized, or at least reduced to one-half the level encountered in conventional LM processing; (3) formulation of the LM emulsion can be modified to allow easy demulsification; (4) leakage of internal, and consequent loss of acid from the electrolytic solution can be minimized and (5) LM extraction and solvent extraction can be advantageously combined into a single process which results in advantages and simplifications to both.
Easier demulsification (item 3 above) follows because leakage becomes less important if the LM contacting operation is reduced to a short residence time step involving only "higher" Cu-concentrations. Therefore, it will be possible to use a membrane formulation which will allow easy demulsification. It may even be possible to essentially eliminate the aqueous/thick emulsion contacting step.
In addition to these advantages, the solvent extraction step helps remove residual emulsion if there is any left from the liquid membrane (LM) extraction step. The LM contacting step is followed by a settler where the bulk of the LM "phase" settles out as a separate emulsion layer. However, in some cases, depending on membrane formulation a small quantity of emulsion in the form of very fine droplets remains behind which should be removed. However, it may actually be desirable to do a minimum of LM/feedwater settling and purposely entrain a substantial amount of emulsion into the subsequent solvent extraction zone. In that case, the solvent extraction acts as a very effective emulsion clean-up step, and allows a major reduction in the size of the conventional LM/feedwater settler.